X-radiation (composed of X-rays) is a form of electromagnetic radiation. X-rays have a wavelength in the range of 10 to 0.01 nanometers, corresponding to frequencies in the range 30×1015 Hz to 30×1018 Hz, and energies in the range 120 eV to 120 keV. X-rays are primarily used for diagnostic radiography and for crystallography.
X-rays are generated by an X-ray tube, a vacuum tube that uses a high voltage to accelerate electrons released by a hot cathode to a high velocity. The high velocity electrons collide with a metal target, the anode, thus creating the X-rays (“Brehmsstralung”). The X-ray photons are emitted in pulses with variable duration.
The detection of X-rays may be based on various methods. The most commonly known methods are a photographic plate and X-ray film.
Since the 1970s, semiconductor detectors have been developed (for example silicon or germanium doped with lithium, Si(Li) or Ge(Li)). X-ray photons impinging on a semiconductor material are converted to electron-hole pairs in the semiconductor material and these charge carriers are collected and generate an electrical signal representative of the impinging X-rays.
State of the art X-ray image sensing is performed with pixels that are either “charge integrating” or “photon counting”. The detection itself can be “direct” (where the X-ray photon creates a packet of secondary electron-hole pairs in the semiconductor material, that are subsequently read by a charge sensitive amplifier) or “indirect” (where the X-ray photon is absorbed in a scintillator material where it creates a flash of secondary, visible light, which secondary light is subsequently detected by a visible light image sensor).
In case of charge integration and indirect detection, an image sensor is used for detection of the secondary visible radiation. The image sensor converts this visible radiation into analog electrical signals (current or voltage). The more radiation impinges on the image sensor over time, the higher the corresponding electrical signal. Charge integrating pixels are simple electronic circuits. These have three or a few more transistors. Many examples exist in the state of the art, of which a very simple one is represented in FIG. 1. The image sensor 10 illustrated comprises a phototransducer such as a photodiode 11, for converting the impinging radiation 12 into an electrical signal such as for example a photocurrent. The thus generated photocurrent integrates on an integrating element such as a capacitor 13. A voltage buffer 14 may be provided between the integrating element 13 and read-out circuitry 15 for reading out a radiation value.
In principle the X-radiation consist of separate X-ray photons, which can be detected as separate pulses. For that reason a photon counting method may be more effective than charge integration for detection of the radiation. However, known counting pixels are complex devices. They have an analog front-end that detects pulses (the front end for example comprising a pulse shaper and a comparator) and an elaborate digital counter. The number of transistors is in the hundreds (100 . . . 1000). One example of such a counting pixel 20 is illustrated in FIG. 2. Every X-ray photon 21 which impinges on a phototransducer such as a photodiode 22 creates a small charge packet with hundreds of electrons, which is shaped to a pulse 25 by a pulse shaper circuit 23. A comparator 24 compares the generated pulses 25 to a reference value Vref. Every time the amplitude of the pulses 25 exceeds the reference value Vref, a first value, for example high or digital one, is assigned to the output signal of the comparator 24, and every time the pulses are below the reference value Vref, a second value, for example low or digital zero, is assigned to the output signal of the comparator 24. This way, a binary signal is generated, comprising a pulse train 26. The pulses in the pulse train 26 are then counted by means of a digital counter 27 in order to provide a count value for the impinging incident photons 21.
It is generally understood that the counting approach is superior over the charge integrating approach in terms of noise. The counting of photons is essentially noise free (apart from the inherent photon shot noise—PSN), whereas in integrating mode, the collected noise charge is contaminated by analog “read noise”.
A further advantage of photon counting is that one can do at the same time energy (or wavelength) discrimination, and thus obtain at the same time images for different bands of photon energies (which is referred to as “color X-ray”, similar to the wavelength discrimination in visible light image sensors).
Nevertheless, it is a disadvantage of counting pixels that they require a huge, area and power consuming, yield killing digital counter.
Perenzoni M., Stoppa D., Malfatti M., and Simoni A. disclose in “A Multi-Spectral Analog Photon Counting Readout Circuit for X-Ray Hybrid Pixel Detectors”, Instrumentation and Measurement Technology Conference 2006, Proceedings of the IEEE, Publication Date: 24-27 Apr. 2006, pages 2003-2006, an all-analog pixel architecture for the readout of X-ray pixel detectors. The pixel comprises a self-triggered reset charge amplifier, three autocalibrated comparators, an energy window identification logic and three analog counters with adjustable range. Perenzoni et al. thus show a pixel that avoids the use of a large digital counter, and replaces such digital counter with an analog domain counter. In their approach, the dynamic range is somewhat reduced due to the use of that analog counter, but still it keeps the capability to count several energy bands separately.
An implementation of a counting pixel with a (linear) analog counter according to the prior art is illustrated in FIG. 3. The X-ray photons 31 which impinge on a phototransducer such as a photodiode 32 create small charge packets with hundreds of electrons, which are shaped to pulses 33 by a pulse shaper circuit 34. A comparator 35 compares the generated pulses 33 to a reference value Vref. Every time the amplitude of the pulses 33 exceeds the reference value Vref, a first value, for example high or digital one, is assigned to the output signal of the comparator 35, and every time the pulses are below the reference value Vref, a second value, for example low or digital zero, is assigned to the output signal of the comparator 35. This way, a binary signal is generated, comprising a pulse train 36. The pulses in the pulse train 36 are then counted by means of an analog counter 37 in order to provide a count value 38 for the impinging incident photons 31.
The analog counter 37 comprises a counting capacitor Cs onto which a signal representative of the number of detected photons is stored. Before counting is started, the counting capacitor Cs is reset to an initial value by means of a reset system, for example comprising a reset transistor (switch) 39. The charge is accumulated on the Capacitor Cs in the feedback loop of a charge transimpedance amplifier (CTIA).
One input port of the CTIA is connected to a reference value Ref; the other input port is connected to a second capacitor Cp via a first switch. The second capacitor Cp itself can be short-circuited via a second switch. The first and second switches are operated alternatively: one opens when the other closes and vice versa. The operation of the first and second switches is controlled by the input signal of the analog counter 37, i.e. by the binary pulse train 36.
Upon reception, by the analog counter 37, of a high pulse of the pulse train 36, the charge packet is stored on the second capacitor Cp, after which it is transferred to the counting capacitor Cs.
The counting is linear because the step height is constant, thus the output voltage 38 is a linear function of the number of pulses. In this actual circuit this is realized by accumulating fixed small charge packets. The fixed charge packets are realized by a fixed voltage over the second capacitor (ΔQ=ΔV*C). In this particular case the voltage step over the second capacitor CP is kept constant by the fact that the virtual ground of the CTIA keeps the voltage on the second capacitor CP independent of the already accumulated value on the counting capacitor Cs.
The ratio Cp/Cs determines the step height in the count signal 38 when counting impinging photons. If Cp or Cs are programmable, the step height can be programmed. Also when the voltage on Cp, or the voltage difference between Cp and Cs can be programmed, the step height is programmable.
R. Ballabriga et al., in “The Medipix3 Prototype, a Pixel Readout Chip Working in Single Photon Counting Mode With Improved Spectrometric Performance”, IEEE Trans Nuclear Science, vol. 54, no. 5 (2007), demonstrated the possibilities of using information of multiple pixels for detecting a same impinging particle. They demonstrated that the accumulated charge of a multiplicity of pixels can be combined to recover the particle's total ionization charge. Key in this operation is the detection of “coincidence”, i.e. of hits that happen at the same moment in neighboring pixels due to a same event.